The present invention relates to a surface acoustic wave device for use in communication system, such as filter, and more particularly, to a surface acoustic wave device provided with an absorber for absorbing and suppressing reflected waves.
Generally, a surface acoustic wave (SAW) device comprises, as shown in FIG. 1, a transmitting, or launching, transducer 2 and a receiving transducer 4, which are formed from interdigitated comb-like multi-electrode elements and disposed on a piezoelectric substrate 6. When an alternating electrical potential is applied to the electrodes of the transmitting transducer 2 through terminal plates 8 and 10 located at opposite side of the transducer 2, an alternating electric field is generated that causes localized vibrations in the substrate material 6. These vibrations give rise to acoustic waves, which propagate along the surface of the substrate in a well defined path orthogonal to the electrodes, and may be detected at any point along the path by the receiving transducer 4. The received acoustic waves can be removed as an electric signal, from terminal plates 12 and 14.
In the SAW device described above, the transmitting transducer 2 launches the surface waves in opposite directions simultaneously while the receiving transducer 4 receives the waves traveling in either direction. This is a significant problem in most SAW devices because in addition to responding to surface waves traveling directly from the transmitting transducer 2 to receiving transducer 4, the transducers respond to surface waves reflected from the ends of the substrate. These reflected surface waves produce unwanted signals, for example, as spurious signal in the time domain and/or ripples of the frequency response domain that distort the main, desired signal, adversely affecting the performance of the SAW device.
There have been proposed various methods for suppressing the reflected waves. One method is, as shown in FIG. 1, to apply a rectangular absorbent material 16, 18 to the edges of the substrate adjacent to each transducer. Since the larger the absorbent material area, the higher the reduction of the surface waves, each absorbent material 16, 18 has a length, measured in the direction of propagation of the surface wave, sufficiently long to absorb the unwanted surface waves launched towards the edges of the substrate and reflected from such edges.
Another method is shown in FIG. 2 in which the side edges are skewed relative to the fingers of transducer combs, and thus relative to the principal acoustic response axis, to change the direction of travel of the reflected surface waves for reducing the influence of the reflected surface wave on the transducer.
Although the above described methods effectively suppress the unwanted waves and reflected waves, the application of absorbent material 16, 18 in the manner described above, or the change of the configuration of the substrate result in high manufacturing cost. More particularly, according to the SAW device of FIG. 1, each of the absorbers 16 and 18 occupies a large space in the substrate 6 to substantially enlarge the size of the SAW device. Thus, it is necessary to provide a substrate having a large size. Furthermore, since the absorbers 16 and 18 cover large area over the substrate, they require a large amount of absorbent material. Also, according to the SAW device of FIG. 2, since the substrate 6 is formed in the shape of parallelogram, a parent plate from which the substrates are cut out may not be fully utilized, thus wasting the substrate material.
These and other conventional SAW devices are disclosed in Toda et al's U.S. Pat. No. 4,090,153 issued May 16, 1978, Drummond's U.S. Pat. No. 4,096,455 issued June 20, 1978, and Yamada et al's Japanese Laid Open Patent Publication No. 100253/1976 (corresponding to U.S. Pat. No. 4,139,791 issued Feb. 13, 1979).
Accordingly, a primary object of the present invention is to provide a SAW device in which the unwanted waves and reflected waves can be effectively suppressed with a small amount of absorbent material.
It is another object of the present invention to provide a SAW device of the above described type which is compact in size and can readily be manufactured at low cost.